1. Field of the Invention
The present invention relates to coatings that inhibit the formation and adhesion of deposits on surfaces that contact hydrocarbon fluids. More particularly, this invention relates to a method and coating system for preventing or reducing the deposition of carbonaceous deposits on surfaces that contact a hydrocarbon fluid at high temperatures.
2. Description of the Related Art
Thermal instability, or in the case of fuels, fuel instability, generally refers to the formation of undesired deposits that occurs when hydrocarbon fluids, such as fuels and lubricating oils, are at high temperatures, generally above about 140xc2x0 C. In the case of fuels, it is generally accepted that there are two distinct mechanisms occurring within two overlapping temperature ranges. In the first mechanism, referred to as the coking process, a generally consistent increase in the rate of formation of coke deposits occurs above temperatures of about 650xc2x0 F. (about 345xc2x0 C.). Coke formation is the result of high levels of hydrocarbon pyrolysis, and eventually limits the usefulness of the fuel. A second mechanism primarily occurs at lower temperatures, generally in the range of about 220xc2x0 F. to about 650xc2x0 F. (about 105xc2x0 C. to about 345xc2x0 C.), and involves oxidation reactions that lead to polymerization and carbonaceous gum deposits.
In the past, the solution to the problem of gum and coke formation was primarily directed toward placing limitations on fuel chemistry and impurities associated with fuels, as disclosed in U.S. Pat. Nos. 2,698,512, 2,959,915 and 3,173,247. However, the propensity for gum and coke formation is increased with certain hydrocarbon fluids for fuels, oils, lubricants, petrochemical processes (plastics and synthetics) and the like, especially those derived from nonpetroleum sources, such as shale and coal, which can exhibit significantly more problems with thermal instability because of their high content of olefins, sulfur and other compounds. The consequences of thermal instability and fuel instability are of even greater significance with developing technology that requires processes and machinery to operate at higher temperatures, as afforded by advances in materials technology. Accordingly, fluid containment articles that are resistant to or prevent the formation of adverse decomposition products and foulants are necessary in applications where thermal instability, including fuel instability, is a problem as a result of exposure to such fluids to high temperatures. Particularly notable applications include the fuel-handling and hydraulic components of gas turbine engines. With the advent of higher engine operation temperatures and the use of fuel as a heat sink, there is an increased likelihood that fluid flow through such components will be restricted or even blocked by carbonaceous deposits.
It has been recognized that deposits can form as a result of a reaction between a hydrocarbon fluid and its containment wall. In U.S. Pat. No. 4,078,604, heat exchangers are provided with thin-walled corrosion-resistant layers of electrodeposited gold or similar corrosion-resistant metals on the walls of the cooling channels in order to make the surfaces corrosion resistant to such storable liquid fuels as red fuming nitric acid. In this case, the wall is protected from corrosion, and the intent is not to prevent deposit formations. Furthermore, gold readily diffuses into other materials at elevated temperatures, and therefore is unsuitable as a protective coating for high temperature applications, e.g., temperatures associated with gum and coke formation.
More recently, coating systems specifically directed to inhibiting the formation and adhesion of carbonaceous deposits have been taught. For example, U.S. Pat. Nos. 5,805,973, 5,891,584, 5,923,944 and 6,156,439, all assigned to the assignee of the present invention and incorporated herein by reference, teach the use of coke barrier coatings (CBCs) that eliminate or modify the surface reactions which lead to formation of thermal instability deposits from hydrocarbon fluids, and reduce adhesion of such deposits. These patents are generally directed to ceramic coatings that are especially capable of reducing deposits at very high temperatures e.g., above 650xc2x0 F. (about 345xc2x0 C.). As an example, U.S. Pat. Nos. 5,805,973 and 5,891,584 disclose coatings that catalyze thermal decomposition in the hydrocarbon fluid to actually promote the formation of coke, which is substantially nonadherent to the coatings.
Many applications exist where there is a particular need for coatings that can significantly reduce the formation and adhesion of carbonaceous deposits at lower temperatures, such as fuel/air heat exchangers, fuel nozzles, oil sumps and other fuel and hydraulic system components of gas turbine engines. For this type of hardware, reductions in hydrocarbon deposits have been achieved with the use of coatings that are not reactive with hydrocarbons. In situations where heat transfer from the containment walls is a major contributor to the fluid temperature, thermally-reflective (low emissivity) coatings that reduce heat transfer to the hydrocarbon fluid have been employed to reduce deposit formation. Notably, the CBC systems taught by U.S. Pat. Nos. 5,805,973, 5,891,584, 5,923,944 and 6,156,439 do not have the correct optical properties, including low emissivity, to function as radiation shields. While CBC systems of the prior art can be combined with low-emissivity coatings, a significant drawback is the additional volume, weight and cost incurred. Accordingly, it would be desirable if an improved coating system were available that reduced the formation of carbonaceous deposits in hydrocarbon fluids at temperatures below about 650xc2x0 F., reduced the adhesion of such deposits, and reduced the temperature of the hydrocarbon fluids.
The present invention provides a coating system and method for reducing the tendency for hydrocarbon fluids, such as fuels and oils, to form carbonaceous deposits that adhere to their containment surfaces. The invention is particularly concerned with the carbonaceous deposits that form at temperatures below about 650xc2x0 F. (about 345xc2x0 C.). According to the invention, a coating system that combines an outermost layer of platinum with a ceramic barrier layer has been shown to significantly reduce the formation of carbonaceous deposits at temperatures between about 220xc2x0 F. and 650xc2x0 F. (about 105xc2x0 C. to about 345xc2x0 C.), as well as reduce the adhesion of such deposits. The platinum outermost layer also serves as a radiation shield to reduce heat transfer from the containment article to the hydrocarbon fluid. The outermost layer is preferably deposited as an extremely thin film by chemical vapor deposition. The barrier layer is deposited to a thickness sufficient to prevent interdiffusion of the platinum outermost layer with the wall on which the platinum outermost layer is deposited.
From the above, it can be seen that the coating system of this invention can be present as a very thin coating, yet performs three distinct functions that reduce hydrocarbon fluid deposits at temperatures below about 650xc2x0 F. Accordingly, the coating system of this invention does not share the disadvantages of volume, weight and cost noted for prior attempts to combine CBCs with low-emissivity coatings.
Other objects and advantages of this invention will be better appreciated from the following detailed description.